After a reservoir or well has been drained or if it is determined that a reservoir does not possess sufficient hydrocarbon reserves, steps are taken to ensure proper abandonment of the reservoir. These steps typically include plugging the annulus and wellbore with cement to isolate the reservoir.
When plugging a hole for abandonment, it is important that the cement plug is robust and completely fills the wellbore and annulus to prevent channeling and ensure longevity of the plug. Channeling occurs if cement does not completely fill the wellbore and annulus, creating pockets that may allow migration of gas. Extreme cases of channeling may result in an oil spill or contamination of nearby aquifers. In order to avoid these extreme cases, if a hole has not been properly isolated remedial cementing may be required, which is both time consuming and expensive.
Channeling complications are common in horizontal holes, deviated holes, and large holes due to inadequate borehole coverage. For example, the drill string may not be centered making it difficult for the cementing tool to completely fill the upper region or the high side of the hole. As a result, channeling on the high side is a common occurrence. In large holes the diameter of drill string may be only a fraction of the diameter of the hole. Due to this size difference the tool may not be able to direct cement to the outer edges of the borehole to create a robust plug. In addition, the hole may be irregularly shaped, such as an egg shape or an oval shape, making sufficient borehole coverage difficult. Further, even if initial channeling is minimized, pockets of contaminated cement and mud may prevent isolation of a reservoir. Furthermore, free water in the cement may migrate to the high side compounding channeling problems.